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J. Kagimoto et al. / Journal of Molecular Liquids 153 (2010) 133–138
ILs based on ammonium cations, such as imidazolium, pyrrolidi-
30 min. As free amino acid is insoluble in the mixture of
acetonitrile and methanol, the solution was filtered to remove
unreacted L-leucine. After the filtrate had been evaporated,
water (50 mL) was added to the solution, giving rise to two
phase separation in which the lower phase was water with
some unreacted L-leucine, and the upper phase was decyl-tri-
n-octylphosphonium leucine [P8,8,8,10][Leu]. After removal of
the upper phase, 50 mL of water was added and the solution
was thoroughly mixed. We repeated this procedure several
times to wash out the water-soluble fraction from the product.
The residual solution was dried in vacuo for 1 day at 70 °C.
nium and pyridinium cations, have been investigated extensively in a
large number of papers [20–23]. Recently, phosphonium cations have
been proposed as component cations of ILs with better thermal and
electrochemical stability than using ammonium cations [11,12,24–27].
Of the phosphonium cations, trihexyl(tetradecyl)phosphonium cation
has been studied most [13,28–31]. Although there have been some
studies of the physico-chemical and thermal properties of ILs prepared
with different phosphonium cations [11,26,27], few comparative
reports exist of phosphonium-type ILs. Detailed systematic analysis
has been necessary for further application of these phosphonium-type
ILs.
In the present study, we prepared hydrophobic AAILs ([Pn,n,n,m
]
2.3. Characterization
[amino acid], where n or m denotes the carbon number of alkyl chains
on the phosphonium cation) by introducing long alkyl chains on the
phosphonium cations.
The structure of AAILs was confirmed by 1H NMR spectroscopy
(500 MHz, JEOL). Tetramethylsilane (TMS) was used as an inner
reference for the 1H NMR measurements.
2. Experimental
2.4. Measurement of physico-chemical and thermal properties of AAILs
2.1. Materials
The glass transition temperature (Tg) and melting temperature
(Tm) of the resulting AAILs were measured using a differential
scanning calorimeter (DSC-6220, Seiko Instruments) in the range
from −140 to +200 °C, with a heating rate of 10 °C min−1. The
decomposition temperature (Tdec) was measured by means of
thermogravimetry/differential thermal analysis, using a TG/DTA 220
(Seiko Instruments) with a scan rate of 10 °C min−1 in N2 atmo-
sphere. The viscosity of our series of AAILs was measured by a cone/
plate viscometer (LVDV-1+, Brookfield). Density was measured by a
density/specific gravity meter (DA-100, Kyoto Electronic Manufac-
turing Co.).
Tri-n-octylphosphine and all amino acids used (L-glycine, L-
alanine, L-leucine, L-isoleucine, L-lysine, L-phenylalanine, L-proline,
L-serine, L-aspartic acid, L-glutamic acid, and L-valine) were pur-
chased from Wako Chem. Co. Tetraoctylphosphonium bromide and
hexylbromide were purchased from Aldrich. Heptylbromide, decyl-
bromide, dodecylbromide, and octadecylbromide were purchased
from Tokyo Chemical Industry Co. Tetra-n-pentyl-phosphonium
bromide, tetra-n-heptylphosphonium bromide, tetra-n-decyl-phos-
phonium bromide, tetra-n-dodecyl-phosphonium bromide and
tetra-n-tetradecylphosphonoium bromide were supplied from
Hokko Chem. Co. 4-Fluoro-L-phenylalanine, and s-2-amino-3-[3-
(trifluoromethyl phenyl)] propionic acid were donated by Asahi
Glass Co.
2.5. Water content of AAILs
Upon adding a small excess of water to each amino acid ionic
liquid, we often observed phase separation. The top phase (AAIL
phase) was pipetted out and the water content was determined using
a TG/DTA 220 (Seiko Instruments).
2.2. Synthesis of amino acid ionic liquids
Amino acid ionic liquids (AAILs) containing asymmetric phospho-
nium cations were prepared in the following three steps: (1)
quaternization of tri-alkylphosphine; (2) anion exchange from halide
anions to hydroxide; and (3) neutralization with amino acids. AAILs
containing symmetric cations were prepared from the corresponding
tetraalkyl phosphonium salts by anion exchange followed by
neutralization with amino acids. As an example, the synthesis
procedure of [P8,8,8,10][Leu] was mentioned as follows:
3. Results and discussion
Most of the [Pn,n,n,m][AA]s synthesized in this study were liquid at
room temperature; the exceptions were [P8,8,8,8][Glu] and [P12,12,12,12
]
[Leu]. When [Pn,n,n,m][AA] had a long alkyl chain (n≥6), these were
hydrophobic and accordingly underwent phase separation with
water. The IL phase sat above the water phase after mixing (Fig. 1),
because their density was less than that of water.
(1) Quaternization of tri-alkylphosphine: Tri-n-octylphosphine
(8.00 g; 0.022 mol) was dissolved in toluene (20 mL) and a
small excess of decylbromide (5.30 g; 0.024 mol) was added.
The resulting solution was stirred at 100 °C for 48 h under N2
atmosphere. Toluene was then removed at 50 °C by evapora-
tion, and the residue was recrystallized in hexane (300 mL) in a
freezer. Tri-n-octyldecylphosphonium bromide ([P8,8,8,10]Br)
was obtained as a white powder after removing the solvent
(yield 80 %).
(2) Anion exchange: [P8,8,8,10]Br (10.0 g) was dissolved in a water/
methanol (1:9 w/w) mixed solvent (total 200 mL). The mixed
solution was passed through an anion-exchange column filled
with AmberliteR IRN-78, so as to collect a dilute solution of tri-
n-octyldecylphosphonium hydroxide ([P8,8,8,10] OH).
(3) Neutralization with amino acids: the [P8,8,8,10] OH aqueous
solution was added to an aqueous solution of slight excess of L-
leucine (2.50 g; 0.019 mol). After gentle mixing for 30 min, the
solvent was evaporated slowly at 50 °C. After most of the
solvent had evaporated, acetonitrile (60 mL) and methanol
(40 mL) were added, and the mixture was stirred vigorously for
3.1. Density
The density of the synthesized AAILs ranges from 0.886 to
0.945 g cm−3 at 25 °C, consistently less than that of water. AAILs
containing phosphonium cations had lower density than those
containing imidazolium cations, due to long alkyl chains on the
phosphonium cation. For example, Del Sesto et al. reported this trend
in the density of [P8,8,8,8][Tf2N] [12] and [bmim][Tf2N] (1.07 and
1.43 g cm−3, respectively) [22]. Long alkyl chains are known to lower
the density of the corresponding ionic liquids. We analyzed the effect
of the number of carbon atoms in the cations on the density of [Pn,n,n,n
]
[Leu], as shown in Fig. 2. The density depends mainly on the number
of carbon atoms, regardless of the symmetry of cations. A similar
tendency has been reported for imidazolium salts, in which an
increase in the alkyl chain in the imidazolium reduces the salt density
[30,32,33].
When the cation was fixed to [P8,8,8,8], the salt density increased in
the following order with respect to the amino acid anions; [Leu] b